It is well known to use packers for the purposes of sealing fluid flow paths in oil and gas wells such as oil and gas production wells and in water wells such as injection wells. For example, it is well known to sealingly mount such downhole packers on a base pipe and to prevent fluid flow along an annulus defined between an outer surface of the base pipe and an inner surface of a wellbore. Such downhole packers may, in particular, be used for isolating production zones in an oil or gas well. Such packers generally include a sealing element which, in use, exerts a sufficient pack-off force on an inner surface of the wellbore so as to provide an effective seal to fluid flow. Known downhole packers may include a deformable non-swellable sealing element which, in use, exerts a radially outward pack-off force on the inner surface of the wellbore in response to axial compression of the sealing element. The sealing element may be axially compressed by a variety of different methods which may include at least one of: the application of hydraulic pressure to a setting tool; the application of pressure into an integral setting chamber of the packer; the application of a set down weight; the application of over pull; and the use of a pyrotechnic method involving the use of slow-burning explosives.
Swellable packers are also known which include a swellable sealing element which, in use, exerts a radially outward pack-off force on the inner surface of the wellbore on exposure of the sealing element to one or more fluids present in the wellbore. Swellable packers are generally mechanically simpler and less complex than deformable non-swellable packers.
In some circumstances, it may be necessary or desirable to retrieve a packer from the wellbore, for example to permit well measurements, observations and/or well intervention operations to be performed. Deformable non-swellable packers are generally retrieved by releasing the mechanism that maintains the pack-off force and then pulling the base pipe out of the wellbore with the packer mounted thereon. The retrieval of deformable non-swellable packers generally requires well intervention to release the mechanism that maintains the pack-off force. Well intervention is, however, relatively expensive.
In the case of a conventional swellable packer, it may not be possible to reverse the swelling of the swellable sealing element to permit easy retrieval of the swellable packer. In order to retrieve such a packer, it is generally necessary to apply a sufficient pulling force to the base pipe to overcome the considerable friction forces which arise between the outer surface of the swollen sealing element and the inner surface of the wellbore. Dragging a packer out of the wellbore in this way may cause disintegration of the sealing element. This may prevent re-use of at least some of the packer parts. In addition, debris may be generated in the wellbore which may be prejudicial to subsequent well measurements, observations and/or intervention operations. Moreover, the friction forces may be considerable, thereby requiring the use of high pull forces. Accordingly, dragging a conventional swellable packer out of a wellbore may be time-consuming and/or expensive. This may be particularly true when trying to retrieve multiple packers mounted on the same base pipe at the same time.